For the same mass, molarity decreases with increasing molar mass.
For example, suppose we weigh out 100 g. For a substance with a molar mass of 50 there are 2 mols. For a substance with a molar mass of 100 that is 1 mol. M = mol/L, therefore, the molar mass of 100 material is 1M while the other one is 2 M when each is dissolved in 1 L of solution.